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</a></div> <!----></nav>  <ul class="sidebar-links"><li><a href="/fe/" aria-current="page" class="sidebar-link">大前端学习路径 + B站资源整合</a></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>HTML</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>CSS</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>JavaScript</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>TypeScript</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ES6</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Vue</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Vue3</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>React</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Webpack</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Node.js</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>性能优化</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading open"><span>手写代码</span> <span class="arrow down"></span></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/fe/handwritten/" aria-current="page" class="sidebar-link">常见JS代码实现</a></li><li><a href="/fe/handwritten/promise.html" aria-current="page" class="active sidebar-link">Promise</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/fe/handwritten/code.html" class="sidebar-link">常见编程题</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="promise"><a href="#promise" class="header-anchor">#</a> Promise</h1> <ul><li><a href="https://juejin.cn/post/6945319439772434469?utm_source=gold_browser_extension" target="_blank" rel="noopener noreferrer">从一道让我失眠的 Promise 面试题开始，深入分析 Promise 实现细节<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul> <blockquote><p>考察宏微任务、EventLoop 之类</p></blockquote> <p>Promise 会有三种状态：</p> <ul><li>Pending 等待</li> <li>Fulfilled 完成</li> <li>Rejected 失败</li></ul> <div class="language-JavaScript extra-class"><pre class="language-javascript"><code>Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">res</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>res<span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>

Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span><span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>

<span class="token comment">// 打印结果：0、1、2、3、4、5、6 </span>
</code></pre></div><p>JS是单线程 =&gt; 高耗时操作引起进程阻塞问题 =&gt; 两种执行模式 =&gt; 同步模式（Synchronous）和异步模式（Asynchronous）</p> <p>异步任务主要分为宏任务与微任务两种:</p> <ul><li>宏任务（Macrotask） 称为 Task， 宏任务是由宿主（浏览器、Node）发起的；</li> <li>微任务（Microtask） 称为 Jobs，而微任务由 JS 自身发起。</li></ul> <p>setTimeout 属于宏任务，而 Promise 是个微任务</p> <p>创建微任务的方式：process.nextTick（ Node 端 ） 与MutationObserver（ 浏览器端 ）</p> <p><a href="https://developer.mozilla.org/zh-CN/docs/Web/API/HTML_DOM_API/Microtask_guide" target="_blank" rel="noopener noreferrer">在 JavaScript 中通过 queueMicrotask() 使用微任务<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <h3 id="实现"><a href="#实现" class="header-anchor">#</a> 实现</h3> <div class="language-JavaScript extra-class"><pre class="language-javascript"><code><span class="token comment">// MyPromise.js</span>

<span class="token comment">// 先定义三个常量表示状态</span>
<span class="token keyword">const</span> <span class="token constant">PENDING</span> <span class="token operator">=</span> <span class="token string">'pending'</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> <span class="token constant">FULFILLED</span> <span class="token operator">=</span> <span class="token string">'fulfilled'</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> <span class="token constant">REJECTED</span> <span class="token operator">=</span> <span class="token string">'rejected'</span><span class="token punctuation">;</span>

<span class="token comment">// 新建 MyPromise 类</span>
<span class="token keyword">class</span> <span class="token class-name">MyPromise</span> <span class="token punctuation">{</span>
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">executor</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token comment">// executor 是一个执行器，进入会立即执行</span>
    <span class="token comment">// 并传入resolve和reject方法</span>
    <span class="token keyword">try</span> <span class="token punctuation">{</span>
      <span class="token function">executor</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>resolve<span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">.</span>reject<span class="token punctuation">)</span>
    <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>error<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">reject</span><span class="token punctuation">(</span>error<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>

  <span class="token comment">// 储存状态的变量，初始值是 pending</span>
  status <span class="token operator">=</span> <span class="token constant">PENDING</span><span class="token punctuation">;</span>
  <span class="token comment">// 成功之后的值</span>
  value <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
  <span class="token comment">// 失败之后的原因</span>
  reason <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>

  <span class="token comment">// 存储成功回调函数</span>
  onFulfilledCallbacks <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token comment">// 存储失败回调函数</span>
  onRejectedCallbacks <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

  <span class="token comment">// 更改成功后的状态</span>
  <span class="token function-variable function">resolve</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token comment">// 只有状态是等待，才执行状态修改</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>status <span class="token operator">===</span> <span class="token constant">PENDING</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token comment">// 状态修改为成功</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span>status <span class="token operator">=</span> <span class="token constant">FULFILLED</span><span class="token punctuation">;</span>
      <span class="token comment">// 保存成功之后的值</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span>value <span class="token operator">=</span> value<span class="token punctuation">;</span>
      <span class="token comment">// resolve里面将所有成功的回调拿出来执行</span>
      <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>onFulfilledCallbacks<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment">// Array.shift() 取出数组第一个元素，然后（）调用，shift不是纯函数，取出后，数组将失去该元素，直到数组为空</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>onFulfilledCallbacks<span class="token punctuation">.</span><span class="token function">shift</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>

  <span class="token comment">// 更改失败后的状态</span>
  <span class="token function-variable function">reject</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">reason</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token comment">// 只有状态是等待，才执行状态修改</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>status <span class="token operator">===</span> <span class="token constant">PENDING</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token comment">// 状态成功为失败</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span>status <span class="token operator">=</span> <span class="token constant">REJECTED</span><span class="token punctuation">;</span>
      <span class="token comment">// 保存失败后的原因</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span>reason <span class="token operator">=</span> reason<span class="token punctuation">;</span>
      <span class="token comment">// resolve里面将所有失败的回调拿出来执行</span>
      <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>onRejectedCallbacks<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>onRejectedCallbacks<span class="token punctuation">.</span><span class="token function">shift</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">(</span>reason<span class="token punctuation">)</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>

  <span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">onFulfilled<span class="token punctuation">,</span> onRejected</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> realOnFulfilled <span class="token operator">=</span> <span class="token keyword">typeof</span> onFulfilled <span class="token operator">===</span> <span class="token string">'function'</span> <span class="token operator">?</span> <span class="token function-variable function">onFulfilled</span> <span class="token operator">:</span> <span class="token parameter">value</span> <span class="token operator">=&gt;</span> value<span class="token punctuation">;</span>
    <span class="token keyword">const</span> realOnRejected <span class="token operator">=</span> <span class="token keyword">typeof</span> onRejected <span class="token operator">===</span> <span class="token string">'function'</span> <span class="token operator">?</span> <span class="token function-variable function">onRejected</span> <span class="token operator">:</span> <span class="token parameter">reason</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span><span class="token keyword">throw</span> reason<span class="token punctuation">}</span><span class="token punctuation">;</span>

    <span class="token comment">// 为了链式调用这里直接创建一个 MyPromise，并在后面 return 出去</span>
    <span class="token keyword">const</span> promise2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyPromise</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
      <span class="token keyword">const</span> <span class="token function-variable function">fulfilledMicrotask</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span>  <span class="token punctuation">{</span>
        <span class="token comment">// 创建一个微任务等待 promise2 完成初始化</span>
        <span class="token function">queueMicrotask</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
          <span class="token keyword">try</span> <span class="token punctuation">{</span>
            <span class="token comment">// 获取成功回调函数的执行结果</span>
            <span class="token keyword">const</span> x <span class="token operator">=</span> <span class="token function">realOnFulfilled</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">// 传入 resolvePromise 集中处理</span>
            <span class="token function">resolvePromise</span><span class="token punctuation">(</span>promise2<span class="token punctuation">,</span> x<span class="token punctuation">,</span> resolve<span class="token punctuation">,</span> reject<span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>error<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token function">reject</span><span class="token punctuation">(</span>error<span class="token punctuation">)</span>
          <span class="token punctuation">}</span> 
        <span class="token punctuation">}</span><span class="token punctuation">)</span>  
      <span class="token punctuation">}</span>

      <span class="token keyword">const</span> <span class="token function-variable function">rejectedMicrotask</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> 
        <span class="token comment">// 创建一个微任务等待 promise2 完成初始化</span>
        <span class="token function">queueMicrotask</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
          <span class="token keyword">try</span> <span class="token punctuation">{</span>
            <span class="token comment">// 调用失败回调，并且把原因返回</span>
            <span class="token keyword">const</span> x <span class="token operator">=</span> <span class="token function">realOnRejected</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>reason<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">// 传入 resolvePromise 集中处理</span>
            <span class="token function">resolvePromise</span><span class="token punctuation">(</span>promise2<span class="token punctuation">,</span> x<span class="token punctuation">,</span> resolve<span class="token punctuation">,</span> reject<span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>error<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token function">reject</span><span class="token punctuation">(</span>error<span class="token punctuation">)</span>
          <span class="token punctuation">}</span> 
        <span class="token punctuation">}</span><span class="token punctuation">)</span> 
      <span class="token punctuation">}</span>
      <span class="token comment">// 判断状态</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>status <span class="token operator">===</span> <span class="token constant">FULFILLED</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token function">fulfilledMicrotask</span><span class="token punctuation">(</span><span class="token punctuation">)</span> 
      <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>status <span class="token operator">===</span> <span class="token constant">REJECTED</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> 
        <span class="token function">rejectedMicrotask</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
      <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>status <span class="token operator">===</span> <span class="token constant">PENDING</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment">// 等待</span>
        <span class="token comment">// 因为不知道后面状态的变化情况，所以将成功回调和失败回调存储起来</span>
        <span class="token comment">// 等到执行成功失败函数的时候再传递</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>onFulfilledCallbacks<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>fulfilledMicrotask<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>onRejectedCallbacks<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>rejectedMicrotask<span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span> 
    
    <span class="token keyword">return</span> promise2<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  <span class="token comment">// resolve 静态方法</span>
  <span class="token keyword">static</span> <span class="token function">resolve</span> <span class="token punctuation">(</span><span class="token parameter">parameter</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// 如果传入 MyPromise 就直接返回</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>parameter <span class="token keyword">instanceof</span> <span class="token class-name">MyPromise</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> parameter<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token comment">// 转成常规方式</span>
    <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">MyPromise</span><span class="token punctuation">(</span><span class="token parameter">resolve</span> <span class="token operator">=&gt;</span>  <span class="token punctuation">{</span>
      <span class="token function">resolve</span><span class="token punctuation">(</span>parameter<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  <span class="token comment">// reject 静态方法</span>
  <span class="token keyword">static</span> <span class="token function">reject</span> <span class="token punctuation">(</span><span class="token parameter">reason</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">MyPromise</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
      <span class="token function">reject</span><span class="token punctuation">(</span>reason<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">resolvePromise</span><span class="token punctuation">(</span><span class="token parameter">promise<span class="token punctuation">,</span> x<span class="token punctuation">,</span> resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 如果相等了，说明return的是自己，抛出类型错误并返回</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>promise <span class="token operator">===</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">reject</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">TypeError</span><span class="token punctuation">(</span><span class="token string">'The promise and the return value are the same'</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> x <span class="token operator">===</span> <span class="token string">'object'</span> <span class="token operator">||</span> <span class="token keyword">typeof</span> x <span class="token operator">===</span> <span class="token string">'function'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// x 为 null 直接返回，走后面的逻辑会报错</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>x <span class="token operator">===</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token function">resolve</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">let</span> then<span class="token punctuation">;</span>
    <span class="token keyword">try</span> <span class="token punctuation">{</span>
      <span class="token comment">// 把 x.then 赋值给 then </span>
      then <span class="token operator">=</span> x<span class="token punctuation">.</span>then<span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>error<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token comment">// 如果取 x.then 的值时抛出错误 error ，则以 error 为据因拒绝 promise</span>
      <span class="token keyword">return</span> <span class="token function">reject</span><span class="token punctuation">(</span>error<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token comment">// 如果 then 是函数</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> then <span class="token operator">===</span> <span class="token string">'function'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">let</span> called <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
      <span class="token keyword">try</span> <span class="token punctuation">{</span>
        <span class="token function">then</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>
          x<span class="token punctuation">,</span> <span class="token comment">// this 指向 x</span>
          <span class="token comment">// 如果 resolvePromise 以值 y 为参数被调用，则运行 [[Resolve]](promise, y)</span>
          <span class="token parameter">y</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
            <span class="token comment">// 如果 resolvePromise 和 rejectPromise 均被调用，</span>
            <span class="token comment">// 或者被同一参数调用了多次，则优先采用首次调用并忽略剩下的调用</span>
            <span class="token comment">// 实现这条需要前面加一个变量 called</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>called<span class="token punctuation">)</span> <span class="token keyword">return</span><span class="token punctuation">;</span>
            called <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
            <span class="token function">resolvePromise</span><span class="token punctuation">(</span>promise<span class="token punctuation">,</span> y<span class="token punctuation">,</span> resolve<span class="token punctuation">,</span> reject<span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token punctuation">}</span><span class="token punctuation">,</span>
          <span class="token comment">// 如果 rejectPromise 以据因 r 为参数被调用，则以据因 r 拒绝 promise</span>
          <span class="token parameter">r</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>called<span class="token punctuation">)</span> <span class="token keyword">return</span><span class="token punctuation">;</span>
            called <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
            <span class="token function">reject</span><span class="token punctuation">(</span>r<span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>error<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment">// 如果调用 then 方法抛出了异常 error：</span>
        <span class="token comment">// 如果 resolvePromise 或 rejectPromise 已经被调用，直接返回</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>called<span class="token punctuation">)</span> <span class="token keyword">return</span><span class="token punctuation">;</span>

        <span class="token comment">// 否则以 error 为据因拒绝 promise</span>
        <span class="token function">reject</span><span class="token punctuation">(</span>error<span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
      <span class="token comment">// 如果 then 不是函数，以 x 为参数执行 promise</span>
      <span class="token function">resolve</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    <span class="token comment">// 如果 x 不为对象或者函数，以 x 为参数执行 promise</span>
    <span class="token function">resolve</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

module<span class="token punctuation">.</span>exports <span class="token operator">=</span> MyPromise<span class="token punctuation">;</span>

</code></pre></div></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">更新时间:</span> <span class="time">3/2/2022, 7:13:49 PM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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